1. Field of Invention
This invention relates to energy conversion and more particularly to methods and apparatus for controlling power drawn from an energy converter operable to convert energy from a physical source into electrical energy and more particularly to using a perturb voltage produced according to a decreasing non-linear function of converter power to control the power drawn from the energy converter.
2. Description of Related Art
Energy conversion devices such as photovoltaic arrays are commonly used to provide power to electrical loads. Often, these loads are direct current (DC) loads such as batteries, for example. In order to efficiently utilize the energy conversion device, maximum power should be provided from the energy conversion device to the load. The maximum power available to be provided to the load depends upon the conditions under which the energy conversion device is operated and in the case of a photovoltaic array, these conditions include the amount of insolation and the temperature of the array, for example. A maximum power point, or voltage at which maximum power may be extracted from the array, is a desirable point at which to operate the array and conventional systems seek to find this point. The maximum power point changes however, due to changes in insolation and due to changes in temperature of the array and thus control systems are employed to constantly seek this point.
One way of seeking the maximum power point is to periodically perturb and observe the power output of the array and then adjust the power demanded from the array accordingly to cause the voltage of the array to be as close as possible to the maximum power point. Typically, such perturb and observe methodologies involve perturbing the present power supplied to the load by a fixed amount and then observing the effect on power supplied by the array and the voltage measured at the array. Perturbing typically involves temporarily increasing or decreasing the power supplied to the load by a fixed amount, for example. If, after increasing power demanded from the array, the change in power is negative and voltage measured at the array drops by a significant amount, too much power is being extracted from the array and the power demand on the array must be reduced, in which case the power drawn by the array is usually reduced by a fixed amount. If the voltage does not change by a significant amount when the power is perturbed, perhaps not enough power is being extracted from the array and the present power drawn from the array must be increased in which case the power demanded from the array is usually increased by a fixed amount.
Changes in the maximum power available from the array can be dramatic and instantaneous, from 200 watts to 2000 watts in a matter of seconds, for example. This situation may occur due to a change in the amount of insolation, such as may occur when a cloud moves or dissipates from a position blocking sunlight shining on the array to a position in which full sunlight is received on the array. With fixed 4 watt power increments, and a perturb and observe period of 50 milliseconds, the time to change the power drawn from the array from 200 watts to 2000 watts would be about 22 seconds. During this period the full available power is not being drawn from the array, resulting in inefficient operation. If the array is operating at higher power levels, say at 1600 watts, and the maximum power available changes to 2000 watts due to a change in the amount of insolation, the time required to change the power drawn from the array from 1600 watts to 2000 watts would be only about 5 seconds. Therefore it can be seen that when the array is operating at lower power levels, the increased time needed to determine the maximum power point with a fixed incremental value can result in inefficient operation of the array.